Electron beam tube with longitudinally partitioned drift tube



Jan. 21, 1964 F. G. HAMMERSAND ETAL 3,

ELECTRON BEAM TUBE WITH LONGITUDINALLY PARTITIONED DRIF TUBE Filed Jan. 5, 1961 :7 mm T iR L M E/cmmo 7'. SCHUMRCHEZ W MW United States Patent 3,119,045 ELEQTRON BEAM TUBE WITH LONGITUDINAL- LY PARTHTIONED DRIFT TUBE Fred G. Hammer-sand, East Petersburg, Pa, and Richard T. Schumacher, San Jose, Qalifi, assignors to Radio Corporation of America, a corporation of Delaware Filed Jan. 3, 1961, Ser. No. 80,394 7 Claims. (Cl. 315-529) The present invention relates to electron beam tubes, and particularly to amplifier tubes having a drift tube between the input and output means.

In a velocity modulation or klystron amplifier tube, for example, an electron beam is projected through the high frequency field of an input cavity resonator or waveguide and velocity modulated with the signal to be amplified, then through a conductive drift tube or shield wherein the beam drifts and becomes density modulated or bunched, and then into or through an output cavity resonator or waveguide in which the electron bunches induce an amplified output signal. It is, of course, well known that any appreciable feedback of signal energy from the output to the input of an amplifier tube can result in regeneration or oscillations, which are undesirable in an amplifier. In the usual klystron with a small round beam the drift tube connecting the input and output resonators is circular in cross section and so small in diameter that the drift tube is a waveguide operating below or beyond cutoff and unwanted energy propagation therethrough does not occur. However, we have found that undesirable feedback of signal energy can occur in ldystron tubes of the traveling wave type, for example, wherein a ribbon shaped electron beam is projected transversely through input and output waveguides and an intermediate drift tube of rectangular cross-section. This is true because in such an electron tube the rectangular drift tube usually has a major dimension which is greater than a half-wavelength at the operating frequency, and hence, is not a Waveguide beyond cutoff. It would appear that such feedback could be avoided by designing the tube structure With a drift tube having a sufliciently small width, that is, less than a half-wavelength at the operating frequency. -lowever, the minimum width of the drift tube in the traveling wave kiystron is limited by the necessity, or at least desirability, of providing a relatively Wide ribbon beam for interaction with the waveguide fields over an extended distance along the waveguides, which makes this solution of the problem impractical.

The object of the present invention is to provide means for preventing substantial propagation of signal energy through a drift tube of an electron beam tube without reducing the desired overall dimensions thereof.

This object is achieved, in accordance with the present invention, by providing at least one longitudinal conductive partition within the drift tube dividing the latter into longitudinal waveguides operating below or beyond cutoff at the klystron operating frequency, and preferably beyond cutoff at the highest frequency at which the electron beam tube is adapted to operate. Preferably, several such partitions in the form of thin metal plates are equally spaced from each other and the narrow side walls of a rectangular metal drift tube to form a number of identical waveguides beyond cutoff having a length at least approximately equal to the width. The exact length is a function of the required attenuation and drift-tube length.

In the accompanying drawing:

FIG. 1 is a perspective View, partly in section on the line ii -1 of FIG. 3, of a traveling wave klystron tube incorporating the present invention;

3,1193%5 Patented Jan. 21, 1964 FIG. 2 is an axial sectional view taken on the line 2-2 of FIG. 1;

FIG. 3 is a transverse sectional View taken on the line 3-3 of FIG. 2; and

FIG. 4 is a transverse sectional view of a modification incorporating the invention.

As an example, the present invention is illustrated in FIGS. 1-3 as embodied in a traveling wave klystron tube 10 having an input rectangular waveguide 12 and an output rectangular waveguide 14 connected by a rectan gular drift tube 16, and an electron gun 18 for projecting a ribbon-shaped electron beam B through the two wave guides and drift tube to a collector 26.

Preferably, each of the waveguides l2 and 14 is of the single or double ridge type. Each waveguide in the example is provided on one broad side with a central ridge formed by two longitudinal ribs 22 spaced from the other side to form therewith a concentrated electric field gap. The ribs 22 are spaced apart to pass the ribbon beam therebetween. The two waveguides 12 and 14 are oppositely directed in parallel relation with closed end portions overlapping and open ends extending in opposite directions, as shown best in FIG. 2. To prevent reflections, the closed end portions are terminated by any suitable resistive or lossy means, such as the blocks 24 of lossy material. Both the Wide sides of each of the two waveguides are formed near the closed ends with central elongated longitudinal beam apertures or slots 26, aligned with each other and with the slots in the other waveguide.

The drift tube 16 is illustrated as an open-ended box made up of two side plates 28 and two end plates 30 joined to the adjacent walls of the two waveguides around the slots 26 therein, to form an electrostatic shield for the beam passing through the slots.

The electron gun 18 may comprise an elongated rectangular cathode 32 of arcuate transverse cross-section heated by a heater 34 and a pair of elongated beam forming electrodes 36 mounted on side of the input waveguide 12, in alignment with the slots 26, within a dielectric gun envelope portion 38.

The collector 29 may be hermetically sealed to the output waveguide around the slot 26 therein by a dielectric seal 4t) as shown, or enclosed within a separate envelope portion like the electron gun 18. The parts of the drift tube 16 are hermetically sealed to each other and to the two waveguides to form part of the vacuum envelope of the tube. The open ends of the input and output waveguides l2 and 14 are hermetically closed by dielectric windows 42 designed to present the minimum of obstruction to the passage of radio frequency waves therethrough, to complete the vacuum envelope.

In tests made with a tube structure similar to that described thus far, with the drift tube 16 and slots 26 unobstructed, we discovered that the tube might have a tendency to oscillate due to coupling between the output and input wave guides. The slightest asymmetry in either the input or output waveguide causes a potential difference to be developed across the drift tube 16, thus exciting the drift tube in the TB mode. The wide dimension of the wave guides 12 and 14 is necessarily greater than a half-wavelength at the operating frequency, in order to propagate the signal waves. Since the wide dimension of the drift tube 16 is greater than that of the Waveguides in the example shown, the drift tube also propagates the signal waves.

In accordance with the present invention, we divide the drift tube 16 into two or more waveguides each operating below or beyond cutoff by providing one or more longitudinal conductive partitions 44 within the drift tube and extending parallel to the direction of electron flow to minimize collection of electrons thereby. Preferably, these partitions are thin parallel plates equally spaced from each other and the side plates 30 to form identical waveguides. The wide dimension of these waveguides, that is, dimension (1 in FIGS. 2 and 3, must be less than a half wavelength at the desired operating frequency to prevent substantial propagation of wave energy through the drift tube 16. In cases where the internal width of the drift tube 16 is substantially less than one wavelength, a single centrally-located partition would probably be sutlicient to prevent feedback.

In the example shown in the drawing, three partitions are provided so that the axial length L of each waveguide is at least equal to the width a. The amount of attenuation P obtained at any frequency by dividing the drift tube 16 into waveguides beyond cutoff is proportional to the length L and inversely proportional to the spacing a between the partitions 44. Moreover, for a given structure, with L and a constant, P increases as the frequency decreases.

The number of partitions 44 provided in a given drift tube is chosen so as to make the distance a less than a half wavelength at any frequency at which the tube is intended to operate. In a very narrow band tube the choice is determined by substantially a single operating frequency. In a wide band tube, such as the traveling wave klystron tube shown in the drawing, the choice is preferably determined by the highest frequency in the operating range of the tube, to avoid the possibility of oscillations at any frequency in that range. It may also be required that the choice be determined by the highest harmonic frequency which has sufficient energy to cause oscillations.

The operation of the tube illustrated in FIGS. 13 is similar to that of the traveling wave klystron disclosed in Ginzton Patent 2,698,398, in which the beam apertures or slots are bridged by spaced parallel grid wires. The use of grid wires tends to reduce feedback through the drift tube. However, the grids formed by the grid wires across the ends of the drift tube are relatively thin and relatively greatly spaced apart, and hence, they do not divide the drift space into separate waveguides beyond cutoff as in the present invention. For this reason, the grid wires must be closely spaced in order to produce much attenuation of signal waves in the drift space, in which case they collect a large fraction of the beam current, and hence, reduce the efficiency and power output of the tube. On the other hand, the loss in efficiency and power introduced by the use of a small number of thin partitions can easily be tolerated.

It will be understood that the present invention is not limited to use in a traveling Wave klystron tube as shown in the drawing, but may be used in any beam tube having a drift tube that can be divided by longitudinal partitions into a plurality of waveguides beyond cutoff. For example, as shown in FIG. 4, in a standing-wave klystron in which a hollow beam B is projected through two resonators and a hollow cylindrical drift space between two concentric cylindrical conductors 52 and 54, the drift space 5t) can be divided by radial partitions 56 into a plurality of waveguides beyond cutoif. Moreover, the invention may be used in tubes other than the velocity modulation type, such as a tetrode having the space between the control grid and the screen grid surrounded by a drift tube.

What is claimed is:

1. An electron beam tube comprising an electron gun for projecting an electron beam along a given path, conductive means adjacent to said gun for modulating said beam with an input signal, conductive means spaced along said path from said modulating means for extracting amplified signal energy from said beam, a conductive drift tube surrounding the portion of said beam path extending between said modulating means and said extracting means,

and conductive partition means dividing said drift tube into waveguides operating below cut-off at the operating frequency of the tube for preventing substantial propagttion of signal energy through said drift tube, said partition means comprising at least one elongated partition in said drift tube extending parallel to the direction of electron flow in said beam, said drift tube having such dimensions that it would propagate signal energy therethrough at the operating frequency if said partition means were omitted.

2. An electron beam tube as in claim 1, wherein said drift tube is rectangular in cross-section with the major dimension greater than a half-wavelength at the operating frequency, and said waveguides are rectangular in crosssection with the major dimension less than a half-wavelength, at the operating frequency.

3. An electron beam tube as in claim 2, wherein the axial length of said waveguides is at least equal to said major dimension.

4. An electron beam tube as in claim 2, wherein said partition means consists of at least two parallel conductive partitions dividing said drift tube into at least three identical waveguides.

5. A traveling wave tube comprising a pair of spaced, parallel, input and output waveguides of rectangular crosssection having aligned longitudinal slots in opposite major walls near the ends thereof, a conductive drift tube of rectangular cross-section surrounding said slots and extending between said waveguides, an elongated electron gun adjacent to said input waveguide for projecting a ribbon beam of electrons through said waveguides and said drift tube, an elongated collector adjacent to said output waveguide for collecting said beam, and conductive partition means dividing said drift tube into rectangular waveguides operating below cut-off at the operating frequency of the tube for preventing substantial propagation of signal energy through said drift tube, said partition means comprising at least one elongated partition in said drift tube extending parallel to the direction of electron flow in said ribbon beam, said drift tube having such dimensions that it would operate as a waveguide above cut-off at the operating frequency if said partition means were omitted.

6. A traveling wave tube as in claim 5, adapted to be operated over a given frequency range, and wherein the major dimension of said last-named waveguides is less than a half-wavelength at the highest operating frequency in said range.

7. An electron beam tube comprising an electron gun for projecting an electron beam along a given path, conductive means adjacent to said gun for modulating said beam with an input signal, conductive means spaced along said path from said modulating means for extracting amplified signal energy from said beam, conductive wall means forming a drift space containing the portion of said beam path extending between said modulating means and said extracting means, and conductive partition means converting said drift space into at least one waveguide operating below cut-off at the operating frequency of the tube for preventing substantial propagation of signal energy through said drift space, said partition means comprising at least one elongated partition in said drift space extending parallel to the direction of electron flow in said beam, said drift space being such that it would propagate signal energy therethrough at the operating frequency if said partition means were omitted.

References Cited in the file of this patent UNITED STATES PATENTS 2,657,329 Wathen Oct. 27, 1953 2,698,398 Ginzton Dec. 28, 1954 2,733,305 Diemer Jan. 31, 1956 2,920,229 Clarke Jan. 5, 1960 2,932,762 Geppert Apr. 12, 1960 

7. AN ELECTRON BEAM TUBE COMPRISING AN ELECTRON GUN FOR PROJECTING AN ELECTRON BEAM ALONG A GIVEN PATH, CONDUCTIVE MEANS ADJACENT TO SAID GUN FOR MODULATING SAID BEAM WITH AN INPUT SIGNAL, CONDUCTIVE MEANS SPACED ALONG SAID PATH FROM SAID MODULATING MEANS FOR EXTRACTING AMPLIFIED SIGNAL ENERGY FROM SAID BEAM, CONDUCTIVE WALL MEANS FORMING A DRIFT SPACE CONTAINING THE PORTION OF SAID BEAM PATH EXTENDING BETWEEN SAID MODULATING MEANS AND SAID EXTRACTING MEANS, AND CONDUCTIVE PARTITION MEANS CONVERTING SAID DRIFT SPACE INTO AT LEAST ONE WAVEGUIDE OPERATING BELOW CUT-OFF AT THE OPERATING FREQUENCY OF THE TUBE FOR PREVENTING SUBSTANTIAL PROPAGATION OF SIGNAL ENERGY THROUGH SAID DRIFT SPACE, SAID PARTITION MEANS COMPRISING AT LEAST ONE ELONGATED PARTITION IN SAID DRIFT SPACE EXTENDING PARALLEL TO THE DIRECTION OF ELECTRON FLOW IN SAID BEAM, SAID DRIFT SPACE BEING SUCH THAT IT WOULD PROPAGATE SIGNAL ENERGY THERETHROUGH AT THE OPERATING FREQUENCY IF SAID PARTITION MEANS WERE OMITTED. 